There is significant interest in the use of stem cells (SCs) as a therapy for myocardial repair after myocar- dial infarction. However, the survival of SCs, after transplantation to the injured myocardium, has been poor and will limit the beneficial effect of these therapies. Understanding how SCs interact with the injured post- ischemic myocardium will provide critical insight that can be used to optimize SC therapies. The pro-inflammatory, pro-apoptotic, and pro-fibrotic milieu of the post-ischemic myocardium creates a hostile microenvironment that affects the function of transplanted SCs, limiting their survival. Cell death can occur due to apoptosis and/or necrosis, and in both scenarios mitochondrial dysfunction has been placed at the center of these events, with dysmodulation of their outer membrane potential, leading to initiation of the apoptotic/necrotic cascade. Thus, the mitochondria appear as a critical organelle that can determine the sur- vival of transplanted SCs. The Principal Investigator has previously shown that reporter gene bioluminescence imaging (BLI) can be used to accurately and longitudinally monitor cell viability non-invasively. Recently, we have developed and validated a reporter gene-based imaging sensor to monitor the biology of the mitochondria, and understand the interaction between transplanted SCs and the ischemic myocardium. The hypothesis of this proposal is that the mitochondrial function of transplanted SCs, as a read-out of the interaction between the post-ischemic myocardium and SCs, can be monitored non-invasively, and that such imaging strategies can be adapted for clinical use. In Aim 1 we will test the hypothesis that the interaction between the post-ischemic myocardium and transplanted SCs can be monitored non-invasively, using mitochondrial function as an indicator of the health of SCs. In Aim 2 we will test the hypothesis that this mitochondrial function monitoring strategy can be applied to a large animal model of cardiac disease. Here, we will use molecular imaging strategies to study the interaction between SCs and the ischemic myocardium. Furthermore, we will provide proof-of-concept for the clinical translation of these strate- gies.